1. Field of the Present Invention
The present invention relates generally to optical switching devices and, more particularly, to an optical switching device for switching an optical signal in a fiberoptic transmission system between two or more possible channels while maximizing signal transmission for each path, and, if desired, for simultaneously switching two separate sets of paths such as the transmitting and receiving legs of an infrared chemical analysis system.
2. Description of the Related Art
One known optical switching device is disclosed in U.S. Pat. No. 5,005,934 entitled "Fiber Optics Channel Selection Device", issued to L. E. Curtiss on Apr. 9, 1991, and assigned to Galileo Electro-Optics Corporation. The '934 device uses two parallel mirrors to switch an optical beam between a first plurality of optical signal channels 62 and a second single signal channel 64. As shown in FIG. 1, the first plurality of channels 62 (only one is shown) include optical fibers 11 and lenses 12 for carrying signals from remote measurement locations and the second single channel 64 includes a lens 14 and an infrared detector 15. An open path 13 connects one of the first and channels 62 with the second channel 64 via first and second parallel mirrors 16, 17 that reflect the beam from the first channel 62 to the second channel 64. A stepper motor 18 rotates the mirrors 16, 17 so that the first mirror 16 is aligned with a desired one of the first signal channels 62 while the second mirror 17 remains aligned with the second signal channel 64.
The '934 device has two significant defects. First, beam spread occurs in the open path 13 between the two lenses 12, 14 and contributes to a significant loss of signal. Second the device only switches one leg of an analysis system (e.g. the receiving leg). In many systems, it is desirable to switch both the transmitted and received signals between various measurement locations.
The beam spread problem is illustrated by FIG. 2 where the open path 13 between the transmitting and receiving lenses 12, 14 is shown as a straight path for clarity. The item at the left of FIG. 2 represents the core of a fiber optic cable 11. The cables used in IR spectroscopic applications usually have quite large core diameters (e.g. 0.6 mm) due to limited signal levels and detector sensitivities encountered in this spectral region. The dashed lines in this figure represent three rays originating from a point at the edge of the core at a distance "r" from the axis of the fiber optic cable 11. Assuming the transmitting and receiving lenses 12, 14 are centered on the axis of the fiber 11, we find that the central ray makes an angle of .alpha. with the axis, where tan .alpha.=r/f. Since the end of the fiber 11 is positioned in the focal plane of the transmitting lens 12, the rays from the point on the edge will form a parallel beam 19 (neglecting optical aberration) making the same angle with the axis. By the time this beam 19 reaches the plane of the receiving lens 14, it will be displaced a distance, d, from the axis, where d=Ltan .alpha.=Lr/f. This results in a loss of signal, with the resultant transmission being proportional to T=D.sup.2 /(D+d).sup.2.
Although devices based on the '934 patent could be used to switch both the transmitted and received legs of an optical system, this is generally not done. There are two probable reasons for this. The first is the relatively high transmission loss of the open path design, discussed above. The second is the fact that the plurality of input channels 62 and the single output channel 64 inherently lie on opposite sides of the switching device. A "double pole, multi-throw" switching device for bi-directional analysis would require either two adjacent switching devices geared together or two devices on either end of a motor shaft that is long enough so that the two sets of fiberoptic connectors forming the plurality of "first optical channels" corresponding to the two directions would be spaced apart and not interfere with each other. Even if this were done, the fiberoptic connections would be awkwardly located.
As indicated by the IR detector 15, the '934 device is usually used to switch the receiving leg of an analysis system. This is acceptable given the usual approach at the transmitting end of the system of spatially dividing the available beam between several transmitting fibers. A bundle of seven fibers, for example, may be illuminated and split into seven channels. It is often possible to do this without sacrificing much usable signal because the throughput of the spectrometer is often considerably greater than that of the optical fibers being used.
Spatial division of the transmitted signal, however, has some disadvantages. FTIR spectrometers which use an interferometer to encode the frequencies are increasingly popular. Accordingly, the portions of the beam being directed to the various fibers may pass through the spectrometer optics at different angles. As a result, they can have different frequency scales, making it difficult to transfer calibrations between different channels of the system.
There is a need, therefore, for an optical switching device which more efficiently couples two or more channels and permits the simultaneous switching of two sets of such channels.